The present invention relates to a method for adjusting the sensitivity of an acceleration detecting device capable of detecting at least one of components, which respectively correspond to the directions of three orthogonal axes, namely, the X-axis, Y-axis, and Z-axis, of an externally acting acceleration, which acts thereon.
In the automobile and machine industries, there has been an increased demand for sensors capable of accurately detecting physical quantities, such as force, acceleration, and magnetism. Especially, compact sensors capable of detecting each of two-dimensional or three-dimensional components of such physical quantities are demanded.
For example, a sensor having a plurality of piezoelectric elements mounted on a flexible plate mounted on a flexible substrate having an operating member is disclosed in the Japanese Unexamined Patent Publication No. 5-26744.
This sensor is configured so that the flexible substrate deforms according to a physical quantity externally acting on the operating member. The direction and magnitude of the externally acting physical quantity are detected by a single acceleration detecting device in a three-dimensional manner on the basis of electric charges that are produced in the piezoelectric elements according to strain due to the deformation of the flexible substrate. FIG. 8 is a perspective diagram illustrating the concept of a coordinate system for a three-axis sensor.
This will be explained hereinbelow by taking an acceleration sensor device, which has an operating member as a weight, as an example of such a sensor device. In the case that an externally acting acceleration a is exerted on an acceleration detecting device as illustrated in FIGS. 2A and 2B, an inertial force f acts on a weight 1 in a direction opposite to the direction of the acceleration a. This inertial force f causes the deformation of a flexible substrate 3 put on the weight 1 and supports 2.
Electric charges according to the direction and magnitude of strain due to the deformation and according to the direction and magnitude of polarization of piezoelectric materials 5 put on the flexible substrate 3 are produced in the piezoelectric materials 5. Thus, the detection of the direction and magnitude of the externally acting acceleration is enabled by outputting the electric charges from upper electrodes 22x, 22y, and 22z, and a lower electrode 21 as electric signals.
The aforementioned acceleration detecting device is configured so that components of the externally acting acceleration, which respectively correspond to the directions of the X-axis, Y-axis, and Z-axis, are detected by a single acceleration detecting device as components. As a result, for example, even when the weight 1 undergoes the influence of the acceleration only in the direction of the Z-axis, as illustrated in FIG. 9B, the strain due to the deformation occurs not only in the piezoelectric element 20x for detecting the X-axis component of the acceleration, but in the piezoelectric element 20y (not shown in FIG. 9B) for detecting the Y-axis component of the acceleration. Consequently, electric charges are produced in the piezoelectric elements 20x and 20y. 
The weight 1, however, does not undergo the influence of the acceleration only in the directions of the X-axis and Y-axis, so that it is necessary to prevent electric outputs of the electric charges produced in the piezoelectric elements 20x and 20y from being electrically outputted therefrom.
Thus, the aforementioned acceleration detecting device employs a method of electrically canceling the produced charges by configuring the pair of piezoelectric elements.
As illustrated in FIGS. 2A and 2B, a piezoelectric device, which corresponds to each of the X-axis, Y-axis, and Z-axis, of the acceleration detecting device comprises at least one pair of piezoelectric elements placed at positions that are symmetric with respect to the weight 1. Because of the symmetric positions of the pair of piezoelectric elements with respect to the weight 1, the amounts of strain of (that is, the amount of the electric charges respectively produced in) these piezoelectric elements of the pair are almost equal to each other. Furthermore, as shown in FIGS. 9A, 9B and 9C, polarization processing having the same magnitude is performed on the piezoelectric elements so that, among piezoelectric materials constituting the piezoelectric elements of the pairs, the piezoelectric materials to be used for detecting the X-axis component and Y-axis component of the acceleration have opposite polarities, and that the piezoelectric materials to be used for detecting the Z-axis component of the acceleration have the same polarity.
When the weight 1 is oscillated by such polarization processing in the direction of the Z-axis as illustrated in FIG. 9B, the electric charges of opposite polarities produced in the piezoelectric elements 20x for detecting the X-axis component and those 20y (not shown) for detecting the Y-axis component are canceled. Thus, no electric signals are outputted from these piezoelectric elements. On the other hand, when the weight 1 is oscillated in the directions of the X-axis or Y-axis the as illustrated in FIG. 9C, the electric charges produced in the piezoelectric elements 20z for detecting the Z-axis component are canceled, so that no electric signals are outputted from these elements 20z. 
However, sometimes, the quantities of electric charges to be produced in the piezoelectric elements of the pair are not equal to each other owing to defective conditions at the time of forming the piezoelectric elements, for instance, variation in the electrode area of the piezoelectric elements, variation in the dielectric constant of the piezoelectric elements, a deviation of the position of the weight, and variation in deformation caused by the bending of the flexible substrate.
In such a case, the electric charges produced in the piezoelectric elements of the pair are not completely canceled but outputted therefrom as electrical signals. Thus, for example, the sensitivity in the direction of the X-axis is indicated despite the fact that the acceleration sensor device undergoes the influence of the acceleration only in the direction of the Z-axis (hereunder, such sensitivity will be referred to as xe2x80x9canother axis noisexe2x80x9d).
It is necessary for ensuring the reliability of the sensor to limit the ratio of the other axis noise to the sensitivity in the direction of an axis to be detected (hereunder, such sensitivity will be referred to as xe2x80x9cprincipal axis sensitivityxe2x80x9d) within a predetermined range (for instance, if the principal axis sensitivity is 100%, the other axis noise should be equal to or less than 5%). On the other hand, it is very difficult to limit the other axis noise within the predetermined range in the process of manufacturing acceleration sensor devices. Thus, there is the necessity for a method for calibrating the other axis noise of the sensor device after manufactured.
The present invention is accomplished in view of the aforementioned circumstances.
Accordingly, an object of the present invention is to provide a method for adjusting the sensitivity of an acceleration detecting device, according to which electrical outputs of piezoelectric elements of a pair of an acceleration detecting device are canceled by making the amounts of electric charges, which are respectively produced in a pair of piezoelectric elements placed at positions symmetric with respect to a weight, equal to each other, to thereby suppress the aforementioned other axis noise.
To achieve the foregoing object, according to the present invention, there is provided a method for adjusting sensitivity of an acceleration sensor device having an acceleration detecting device consisting of a pair of piezoelectric elements for detecting an externally acting acceleration. This method includes the step of changing an electric output of the piezoelectric elements by changing electrostatic capacity of the acceleration sensor device. In the case of this method, the acceleration sensor device having an acceleration detecting device may comprises a weight, a support that is installed around said weight and has a hollow portion, a flexible substrate positioned on the support so that the weight is suspended in the hollow portion of said support, and a pair of piezoelectric elements.
In the case of the method of the present invention, if the acceleration detecting device is adapted to detect an acceleration component in the X-direction or Y-direction, the electrostatic capacity may be changed so that the difference between the absolute values of excitation outputs respectively generated in the piezoelectric elements of the acceleration detecting device becomes small when the acceleration sensor device is oscillated in the Z-direction. If the acceleration detecting device is adapted to detect an acceleration component in the Z-direction, the electrostatic capacity may be changed so that the difference between the absolute values of excitation outputs respectively generated in the piezoelectric elements of the acceleration detecting device becomes small when the acceleration sensor device is oscillated in the X-direction or Y-direction.
In the case of the method of the present invention, two pairs of the piezoelectric elements may be provided in said acceleration sensor device correspondingly to two orthogonal axes. In the case of the method of the present invention, three pairs of the piezoelectric elements may be provided in said acceleration sensor device correspondingly to three orthogonal axes.
Further, in the case of the method of the present invention, if the acceleration detecting device consisting of the pair of piezoelectric elements is adapted to detect an acceleration component in the X-direction, the electrostatic capacity may be changed so that the shapes of the piezoelectric elements are symmetric with respect to the X-axis and so that the difference between the absolute values of excitation outputs respectively generated in the piezoelectric elements of the acceleration detecting device becomes small when the acceleration sensor device is oscillated in the Z-direction. If the acceleration detecting device consisting of the pair of piezoelectric elements is adapted to detect an acceleration component in the Y-direction, the electrostatic capacity may be changed so that the difference between the absolute values of excitation outputs respectively generated in the piezoelectric elements of the acceleration detecting device becomes small when the acceleration sensor device is oscillated in the Z-direction.
According to the present invention, the electrodes respectively corresponding to the piezoelectric elements of the pair for detecting an acceleration component in the Z-direction which should undergo trimming processing may be determined according to whether or not an excitation output generated in the piezoelectric elements for detecting an acceleration component in the Z-direction and an excitation output generated in the piezoelectric elements for detecting an acceleration component in the X-direction are different in sign from each other when the acceleration sensor device is oscillated in the X-direction, and according to whether or not an excitation output generated in the piezoelectric elements for detecting an acceleration component in the Z-direction and an excitation output generated in the piezoelectric elements for detecting an acceleration component in the Y-direction are different in sign from each other when the acceleration sensor device is oscillated in the Y-direction.
Further, according to the present invention, an adjusting capacitance, which is connected to an electrode corresponding to each of the piezoelectric elements of the pair, may be formed at a portion which corresponds to the support of a piezoelectric material of a corresponding one of the piezoelectric elements of the pair. Furthermore, trimming processing may be performed on the adjusting capacitance instead of performing trimming processing on the electrode. In this case, preferably, the adjusting capacitance is connected to an electrode corresponding to the piezoelectric element, which has a larger excitation output, of the pair between the piezoelectric elements of the pair, whose excitation outputs are made to be different from each other.
Furthermore, according to the present invention, a part of the electrode may be placed at a portion, which corresponds to said support or to an operating member, of a piezoelectric material corresponding to one of the piezoelectric element of the pair. Further, trimming processing may be performed at the part of the electrode. In this case, preferably, an electrode corresponding to the piezoelectric element which has a larger excitation output between the pair of the piezoelectric elements whose excitation outputs are made to be different from each other, has a part placed on a portion which corresponds to the support or to the operating member of the piezoelectric material. Moreover, the electrode may be formed like a comb and may have bottom land portions each formed on the portion which corresponds to the support or to the operating member of the piezoelectric material, and may have tooth-like portions each projecting from a corresponding one of the bottom land portions and each extending from a portion which corresponds to the hollow portion of the piezoelectric material. The bottom land portions provided between adjacent ones of said tooth-like portions maybe cut by performing trimming processing.
Furthermore, trimming processing may be performed on a portion which is provided at the side of the operating member or of the support of the electrode along the circumference of a circle, the center of which is an origin (O). Alternatively, trimming processing may be performed on the electrode along a line segment connecting an origin (O) to a point provided on the circumference of a circle, the center of which is the origin (O).